1. Field of the Invention
This invention relates to electronic digital thermometers and, more particularly, to an electronic digital thermometer capable of sensing and numerically displaying temperature in either degrees Fahrenheit or degrees Celsius.
2. Description of the Prior Art
U.S. Pat. No. 4,117,722, issued to P. M. Helmstetter on Oct. 3, 1978, describes apparatus wherein a thermistor is used to sense temperature. In the apparatus, the thermistor is also associated with circuitry to provide a temperature related current to a capacitor to charge the capacitor to a predetermined threshold level, after which the capacitor voltage is rapidly reduced to a predetermined lower level. The rise time of the voltage across the capacitor is dependent on the magnitude of the temperature related current and, therefore, the charging and discharging repeats itself with a frequency related to the temperature at the site of the thermistor. The threshold circuitry includes a flip-flop which provides a signal at such frequency and the signal is coupled to a counter by a gate which is open for a first period of time if a measure of temperature in degrees Celsius is desired or for a second period of time if a measure of temperature in degrees Fahrenheit is desired. The first and second predetermined periods are related to the nine-fifths ratio which relates Fahrenheit and Celsius degrees. The counter includes a selectable preset count which is used to compensate for the 32.degree. offset between the Fahrenheit and Celsius temperature scales.
The known prior art also includes the digital thermometer disclosed in copending patent application Ser. No. 093,841, filed on Nov. 13, 1979, in the name of Clinton L. Pinkham and assigned to the assignee of this application. The copending application describes a Fahrenheit/Celsius digital thermometer in which the capacitor charging current has a value proportional to the difference between a temperature related current and a second predetermined current. The predetermined current may be selected so that the frequency at which the capacitor is charged and discharged accurately represents the temperature sensed in either the Celsius or Fahrenheit scale systems. Another feature of the apparatus of the copending application resides in that Celsius or Fahrenheit indications can be provided by changing, in the apparatus, the value of the capacitance provided by the capacitor and/or the magnitude of the voltage range allowed across the capacitor.